Films made from polyester represented by polyethylene terephthalate resin have been extensively applied to various uses in view of the mechanical property, heat resistance and the like. However, they are not suitable for some uses because of the inferiority in flexibility and molding processibility. On the other hand, since films made from polyamide represented by 6-nylon are superior in flexibility, pinhole resistance and gas barrier property, they are applied to many uses such as food packaging materials and the like. However, due to poor dimensional stability against moisture absorption, they cannot be easily applied to food packaging uses and industrial uses involving boiling treatment and retort treatment.
Therefore, a polyester film having flexibility, which is one of the features of polyamide film, has been considered. Most of these films acquire flexibility based on the use of a polyester copolymer in a part or the entirety of the constituent resin. However, since they show degraded strength and elastic modulus, which are mechanical properties, problems may occur during post-processing such as printing and the like.
In view of the above, a flexible film made from crystalline polyester has been studied and, for example, films comprising polyethylene terephthalate resin and polybutylene terephthalate resin are known (Patent References 1, 2, 3). In these films, prevention of the incidence of problems during post-processing has been tried by decreasing thermal shrinkage by conducting heat setting after completion of biaxial orientation. Since the difference in the melting points between polyethylene terephthalate resin and polybutylene terephthalate resin is about 30° C., heat shrinkage cannot be suppressed sufficiently, which in turn necessitates many limitations during post-processing.
Moreover, since these film are free of an easily adhesive coating layer, problems occur in that easily adhesive property of ink becomes degraded, stability of gas barrier property becomes defective after boiling treatment and the like, since a vapor-deposited layer formed of metal or inorganic oxide shows poor adhesion and so forth. In addition, problems of whitening and the like occur unless conditions for forming an easily adhesive coating layer are optimized.                Patent Reference 1: JP-A-2002-037993        Patent Reference 2: JP-A-2002-179892        Patent Reference 3: JP-A-2002-321277        